JP2023523357A - Multi-fiber light guide, device with multi-fiber light guide, and manufacturing method thereof - Google Patents

Abstract

A multi-fiber light guide (1) comprising a number of light guiding fibers (3), each fiber (3) comprising an elongated glass core (5), wherein said core (5) comprises a glass cladding ( 7), said cladding (7) forming a rigid continuous glass member (9) with said core (5), and said core (5) having a higher refractive index than said cladding (7). so that light can be guided by total internal reflection along the glass core (5), and the glass member (9) of the multi-fiber light guide (1) has two butting surfaces (10, 11) , said core (5) terminating in both abutment surfaces (10, 11) such that light passes along said core (5) from one abutment surface (10) to the other abutment surface (11) and the glass of the core (5) and the glass of the cladding (7) contain alkali ions, and the alkali ions of the glasses are distributed on each of the abutment surfaces (10, 11 ) are at least partially exchanged by higher atomic number alkali ions in the ion exchange layer (13) at the abutment surfaces (10, 11). A multi-fiber light guide (1) that is compressively stressed at .

Description

This disclosure relates generally to fiber optic light guides. Specifically, the present disclosure relates to fiber optics comprising multiple fused optical guiding fibers.

One type of fiber optic light guide composed of fused optical fibers is a fiber optic faceplate. US Patent Application Publication No. 2008/0069505 (US2008/0069505 A1) discloses a pixelated display covered with a fiber optic faceplate mounted away from the pixels. Further, US2006/0008220 A1 describes an optical low-pass filter using a fiber faceplate.

A fiber optic plate used in front of the display can serve to increase or decrease the viewing angle, or generally shift the image plane with respect to the display. At the faceplate the individual fibers are fused together with the longitudinal direction of the fibers transverse to the plane of the faceplate. Because the fibers are fused together perpendicular to the plane of the faceplate, the fused regions can result in areas of weakness extending transversely through the plate. Furthermore, glasses for fiber optic light guides are chosen for their optical properties. A suitable glass is therefore not necessarily the strongest glass in terms of mechanical stability. Therefore, rigid multi-fiber light guides are prone to breakage under mechanical stress, especially impact or shock.

U.S. Patent Application Publication No. 2008/0069505 U.S. Patent Application Publication No. 2006/0008220

It is therefore an object of this disclosure to provide a multi-fiber light guide with improved mechanical stability to avoid breakage. Said problem is solved by the subject matter of the independent claims. The dependent claims define advantageous refinements.

Accordingly, this disclosure provides a multi-fiber light guide that includes multiple light guiding fibers, each fiber including an elongated glass core. The core is surrounded by a glass cladding so that the cladding forms a rigid continuous glass member with the core. Since the core has a higher refractive index than the cladding, light can be guided along the glass core by total internal reflection. The glass member (9) of the multi-fiber light guide has two mating surfaces and the core terminates on both mating surfaces so that light can be guided along the core from one mating surface to the other. In addition, the core glass and the cladding glass contain alkali ions. The alkali ions of the glass are at least partially exchanged with higher atomic number alkali ions in the ion exchange layers at each abutment surface. Alkali ions thus exchanged within the ion exchange layer impart compressive stress at the abutment surfaces. Accordingly, a multi-fiber light guide with chemically strengthened butting surfaces is provided. Surprisingly, it has been found that a continuous layer with compressive stress can be formed at the abutting surfaces even though the core and cladding glasses differ in their chemical composition.

The ion exchange layer extends into the glass of the core, cladding, and/or shell surrounding the core and cladding. Advantageously, the ion exchange layer penetrates into the glass core, glass cladding and/or glass shell. Advantageously, the penetration depth of the ion-exchange layer in the glass core is different, especially smaller, than the penetration depth of the ion-exchange layer in the glass cladding. The shell provides additional mechanical stability against stresses from outside the multi-fiber light guide, especially against laterally induced stresses.

According to one embodiment, the glass members forming the multi-fiber light guide are respectively plates or discs. In this embodiment with plate-like or disk-like glass members, the abutting surfaces of the light guides are typically closer together than the longest lateral dimension of the glass members. Therefore, the thickness of the disc or plate is less than its longest lateral dimension. Since the thickness of the disc defines the length of the fiber, the length of the guiding fiber is likewise less than the longest lateral dimension. This kind of multi-fiber light guide is called a faceplate. Preferably, this condition also holds for the shortest lateral dimension of the disc or plate. Therefore, preferably the thickness of the plate, or the length of the fiber or core, is less than the shortest lateral dimension of the disc.

According to a further embodiment, the multi-fiber light guide has an elongated shape. In this embodiment, the fiber length is greater than the longest lateral dimension of the glass member.

The multi-fiber light guide, its manufacturing method, and the device containing the multi-fiber light guide are described in detail below with reference to the drawings.

FIG. 1 shows a multi-fiber light guide in cross-section. FIG. 2 shows a photographic image of the mating surfaces of the multi-fiber light guide. FIG. 3 shows one example of a light guide with light absorbing members. FIG. 4 shows one example of a light guide with light absorbing members. FIG. 5 shows a multi-fiber light guide with a surrounding shell. FIG. 6 shows a graph of the transmittance of a chemically enhanced light guide compared to an unenhanced light guide. FIG. 7 shows a sensor arrangement with a multi-fiber light guide. FIG. 8 shows a multi-fiber light guide shaped as a dental rod. FIG. 9 shows the steps of a method for manufacturing a multi-fiber light guide. FIG. 10 shows the steps of a method for manufacturing a multi-fiber light guide. FIG. 11 shows the steps of a method for manufacturing a multi-fiber light guide. FIG. 12 shows the steps of a method for manufacturing a multi-fiber light guide. FIG. 13 shows an assembly with glass members.

FIG. 1 shows an example of a multi-fiber light guide 1 having a disc or plate like shape. In this embodiment the width w or its lateral dimension is generally greater than the thickness d or length of the light guide 1 .

In general, the light guide 1 contains multiple light guiding fibers 3 . Without being limited to a particular embodiment, the fibers 3 are arranged side by side such that their longitudinal directions are generally parallel to each other. Fiber 3 comprises core 5 embedded in cladding 7 . The core 5 and cladding 7 glasses are fused together to form a solid glass member 9 in which the core 5 and cladding 7 are continuous. Preferably, the individual fibers are of microscopic size, allowing optical signals to be transferred with high spatial resolution. According to one non-limiting embodiment, the pitch or center-to-center distance c of adjacent light guiding fibers 3 is between 2 μm and 500 μm, preferably up to 200 μm. Preferably, the width of the cladding 7 between the cores 5 is above 0.5 μm, preferably above 1 μm, preferably above 1.5 μm. In some cases, the width of the cladding 7 between cores 5 can exceed 2 μm.

The light guiding fiber 3 terminates on both opposing abutment surfaces 10 , 11 of the glass member 9 . Thus, the fiber 3 connects the mating surfaces 10, 11 such that the fiber 3 guides light received at one of the mating surfaces 10, 11 and emits light at the respective opposing mating surfaces 11, 10. . On the one hand the diameter of the glass member 9 or the multi-fiber light guide 1 is greater than 5 mm, preferably greater than 15 mm, preferably greater than 25 mm and/or less than 50 mm, preferably less than 40 mm, preferably less than 30 mm. good. On the other hand, the thickness of the glass member 9 or the multi-fiber light guide 1 is greater than 0.3 mm, preferably greater than 0.6 mm, preferably greater than 1 mm and/or less than 2 mm, preferably less than 1.6 mm, Preferably it may be less than 1.3 mm. Thus, the glass member 9 or multi-fiber light guide 1 may have a diameter-to-thickness ratio of 2-170, preferably 16-25. Such dimensions provide a wide field of application for the multi-fiber light guide 1, eg cover glass for electronics, and ensure sufficient mechanical stability.

The interface between the core 5 and the cladding 7 extends transversely to the abutment surfaces 10, 11 and may constitute weak zones. In order to improve the strength of the multi-fiber light guide 1 , especially in weak areas, the mating surfaces 10 , 11 are subjected to an ion exchange process to form an ion exchange layer 13 on the surfaces of the mating surfaces 10 , 11 .

In order to achieve a continuous ion exchange layer, it is preferred that both the core 5 glass and the cladding 7 glass have compositions with alkali oxides. Preferably the total alkali content, especially the combined alkali content, of the glass of the core 5 and/or cladding 7, or optionally also of the shell 17, is such that the amount of alkali ions with smaller ionic radii gives rise to larger ionic radii. If the amount of alkali ions present is more than 3% by weight, preferably more than 5% by weight. Beneficially, the ratio in mol % of the amount of alkali ions with a smaller ionic radius to the amount of alkali ions with a larger ionic radius is above 1.5, preferably above 2, more preferably Exceeds 2.2. In some cases alkali ions with large ionic radii are absent, or rather their amount is very low. Thanks to the alkali content above 5%, a sufficient and in particular improved chemical exchange can be achieved, so that a sufficient amount of compressive stress can be imparted in the glass of the core 5, cladding 7 and/or shell 17. Ideally, the core 5 glass has a lower alkali content than the cladding 7 glass. Thus, the compressive stress imparted in core 5 and/or cladding 7 can be individually adjusted for a particular application and/or glass composition.

In the ion exchange process, the alkali ions of the glass are partially exchanged by heavier alkali ions with larger ionic radii. Thus, the ions impart compressive stress to the glass within the ion exchange layer 13 . According to the ion exchange process, Na ions are leached out of the glass of core 5 and/or cladding 7 by diffusion. Typically, and depending on the time taken for the exchange process, alkali ions with larger atomic radii, mainly K ions, penetrate into the glass of core 5 and/or cladding 7 to form ion-exchange layer 13. Form. The penetration depth of alkali ions with larger atomic radii in the glass of core 5 and/or cladding 7 is preferably above 2 μm, more preferably above 2.5 μm after 10 hours of ion exchange. After 20 hours of ion exchange, in particular after more than 20 hours, the penetration depth can be from 3 μm to 35 μm or more. A penetration depth of more than 1.5 μm is preferred in order to be able to create a sufficiently high compressive stress at the surface and allow a subsequent polishing step. It may be possible that the greater penetration depth of alkali ions in the core 5 and thus the DoL (depth of the ion exchange layer) is greater than that of the cladding 7 . Beneficially, the penetration depth of alkali ions with a larger atomic radius in the core 5 is smaller than the penetration depth of alkali ions with a larger atomic radius in the cladding 7 . Preferably, the difference in penetration depth of alkali ions with larger atomic radii in core 5 and cladding 7 or the difference in DoL in core and cladding, respectively, is between 2 and 20 times after 20 or 30 hours, in particular it is at least ten times.

Thus, the desired optical properties of the core 5 can be reduced during the ion exchange process and because the effects on the optical properties due to the ion exchange, e.g. stress birefringence of the core 5 glass, are very low or even absent. It can be retained after ion exchange. Simultaneously with compressive stress, at least enhanced mechanical stability can be imparted to the cladding. In this way, cracks that may grow and/or propagate through the cladding 7 and particularly between the cores 5 or fibers, or even across the entire glass member 9 or multi-fiber light guide 1, are prevented or at least can be reduced. Furthermore, due to different levels of compressive stress and/or glass composition in core 5 and cladding 7, crack propagation may be impeded at their interface. In general, ion exchange occurs across the surface of the glass member 9 when the ion exchange layer 13 is produced by completely immersing the glass member in a salt bath. A continuous lateral compression of the glass member 9 can thus be achieved. Furthermore, cross-diffusion between the core 5 and the cladding 7 is possible at the surface or ion exchange layer 13 . This can result in a gradient in depth compression, which can increase from deeper glass regions to planes (10, 11). Therefore, preferably the depth compression at the planes (10, 11) is stronger than in the deeper regions of the glass. Typically, according to the preferred embodiment, the ion exchange layer 13 also extends along a peripheral surface or shell 17 that connects and extends between the two abutting surfaces 10,11.

FIG. 2 shows a photographic image of the mating surfaces 10, 11 of the multi-fiber light guide 1. FIG. Generally, core 5 may have a hexagonal cross-section, as in the example of FIG. Further, generally and without limitation to the cross-sectional shape of the cores 5, the cores 5 can be arranged in a hexagonal pattern. Those features can be advantageous in several respects. If the image of the pixel display is transferred from one mating surface to the opposing surface, the hexagonal pattern can help avoid moire patterns at the mating surface from which the light emerges. Furthermore, the honeycomb-like structure of the interface between the core 5 and the cladding can enhance mechanical stability.

According to a further embodiment, which is also realized in the example of FIG. 2, the glass of the cladding 7 can be at least partially light absorbing. This is generally advantageous for reducing crosstalk or light seepage between fibers 3 . According to one embodiment, which is also realized in the embodiment of FIG. 2, the glass of the cladding 7 is black glass. However, the glass may be partially absorbent, as is the case with colored glass. In this case, the glass has a spectrally dependent and spectrally varying absorption. For example, the glass may be brown, red or purple.

Another way to reduce crosstalk between fibers 3 is to provide additional absorbing fibers within the glass member 9, which are placed between the cores 5 of the guiding fibers. Two examples of those embodiments are shown in FIGS. FIG. 3 shows a stack of individual light guiding fibers 3 and light absorbing fibers 15 arranged in the gaps between said light guiding fibers 3 . The stack of fibers 3, 15 can be fused together to form a rigid rod. A multi-fiber light guide 1 can then be manufactured by cutting a section of desired dimensions from the rod.

FIG. 4 shows an example of a multi-fiber light guide 1 in which the fibers are fused together to form a rigid continuous glass member 9 . In this example the cores 5 are arranged in a square pattern. Light-absorbing fibers 15 are arranged at every two intersections of the four cores 5 . Both core 5 and light absorbing fiber 15 are surrounded by cladding 7 . Additional light-absorbing fibers 15 within the rigid glass member 9 provide extra-wall absorption to reduce crosstalk or block light passing diagonally through the multi-fiber light guide 1 .

In this example, the multi-fiber light guide 1 comprises three different glasses: the glass of the core 5 , the glass of the cladding 7 and the glass of the light absorbing fiber 15 . The glass of the light-absorbing fiber 15 may be similar to one of the other glasses, but contains pigments or dyes, such as colored ions, that absorb light.

FIG. 5 shows a further embodiment of the multi-fiber light guide 1. FIG. This light guide comprises a peripheral shell 17 extending around a central arrangement of fibers 3 . In particular, it is advantageous if this shell 17 is also made of glass and forms an integral part of the glass member 9 of the light guide 1 . The glass member 9 thus comprises a glass shell 17 which surrounds the bundled fibers 3 or the cladding 7 in which the core 5 is embedded, respectively. Thus, the shell also forms the perimeter of the abutment surfaces 10,11.

According to a refinement, the glass of this shell 17, in particular the composition of the glass of the shell 17, preferably the alkali content of the shell 17, is similar to the glass, composition and/or alkali content of the core 5 and the cladding 7. can be different. Thus, in this embodiment, the glass member 9 comprises three different glasses: the core 5 glass, the cladding 7 glass and the shell 17 glass. Furthermore, the glass shell 17 can be chemically strengthened like the core 5 and the cladding 7 to form an ion exchange layer 13 at the abutment surfaces 10, 11 by ion exchange, which preferably comprises the glass member 9 or It is advantageous if it extends continuously over the entire surface of the multi-fiber light guide 1, in particular over the abutment surfaces 10,11.

Advantageously, the width of the shell 17 is greater than the width of the cladding 7, in particular the width of the cladding 7 between the cores 5, in order to improve the mechanical strength of the glass member 9 or the multi-fiber light guide 1 at least in the lateral direction. . The preferred width of the shell is greater than 1%, preferably greater than 2%, preferably greater than 4% of the overall outer diameter of the glass member 9 or multi-fiber light guide 1 and/or the glass member 9 or multi-fiber light guide 1. less than 10%, preferably less than 8%, preferably less than 6% of the total outer diameter of the fiber light guide 1; In some special cases, such as in the case of polished glass member 9 , the width of shell 17 can be less than 1% of the outer diameter of glass member 9 .

The improved mechanical strength of multi-fiber light guides in the form of faceplates was investigated with a ball drop test. Faceplate samples of chemically strengthened faceplates were compared to faceplates that were not chemically strengthened. All samples had a diameter of approximately 13 mm. The sample was placed on a steel plate without a damping material. A steel ball was dropped vertically onto the sample. The test was repeated with increasing weight of the steel ball until the sample failed. Results are shown in the table below:

As is evident from the table above, the mechanical strength against impact or shock is significantly increased by chemical strengthening. Nearly 80% of the chemically strengthened samples survive the 1 g ball drop test, while more than half of the unreinforced samples already break at 1 g weight.

Moreover, ion exchange for chemical strengthening does not adversely affect the optical properties of the multi-fiber light guide.

FIG. 6 shows a graph of the transmittance of a chemically enhanced light guide compared to an unenhanced light guide. For comparison, the transmittance of the multi-fiber light guide was measured before enhancement. The light guide was then subjected to ion exchange for 20 hours. After this ion exchange procedure and chemical strengthening, the permeability measurements were repeated. Curve (b) represents the permeability before ion exchange and curve (a) represents the permeability after the ion exchange step or chemical strengthening, respectively. As can be seen from the curves, the transmittance remains largely unaffected. Surprisingly, the transmittance is even slightly higher after ion exchange, which can be attributed to the change in refractive index in the ion exchange layer 13 .

The multi-fiber light guide 1 according to this disclosure is particularly suitable as a window for optical sensors. Thus, according to one embodiment there is provided an optical sensor structure comprising a sensor element, eg a photodiode, said sensor element encased in a housing comprising a multi-fiber light guide according to this disclosure. A multi-fiber light guide is arranged to direct light from outside the housing to the sensor element. In order to use a multi-fiber light guide as a window it is further particularly advantageous if the sensor structure also contains a light source for interrogating and illuminating the object to be detected by the sensor. In this case, the light guide can effectively suppress the direct signal path from the light source to the sensor due to internal reflections at the outer abutment surfaces of the window. This suppression allows the use of a single common window for both the light source and the sensor. Therefore, in a refinement of this embodiment, the sensor structure comprises a light source, preferably a semiconductor light source, such as a light emitting diode (LED) or a laser diode (LD), in a housing, wherein the light source and sensor element are multi-fiber The light guides are arranged such that the light source emits light through the multi-fiber light guide and the sensor element receives light through the same multi-fiber light guide.

FIG. 7 shows such a sensor structure with a multi-fiber light guide 1. FIG. The sensor structure includes housing 22 . A multi-fiber light guide 1, preferably in the form of a faceplate, forms part of the housing 22 so that light can be transmitted inside the housing and vice versa. Sensor structure 20 includes sensor element 24 . A light source 26 is also included in the preferred embodiment. Sensor element 24 and light source 26 are mounted on a common printed circuit board 28 facing multi-fiber light guide 1 . As shown, optical sensor structure 20 may also include more than one light source 26 . In the example shown, sensor element 22 is positioned between two light sources 26 .

To illustrate the action of the faceplate as a window for sensor element 24 and light source 26, two rays 30, 31 are shown. Ray 31 is a possible path if the window in housing 22 is a simple transparent plate. Thus, the light ray 31 can be reflected by the outer surface of the window and then directly impinge on the sensor element 24 without interaction within the external interaction area 32 . Alternatively, if the multi-fiber light guide 1 is used as a window, the light rays 31 are directed within the light guide fibers 3 to the outer butting surfaces. Even if the light ray is reflected on the outer surface, it can be guided back within the fiber 3 and thus reach the sensor member. In this way unwanted background signals are minimized. After being emitted from the sensor structure 22 , the light rays interact with the external interaction area 32 and can re-enter the housing 22 through the light guide 1 and finally reach the sensor member 24 .

It may be necessary to use separate windows for light source 26 and sensor element 24 to avoid background noise caused by light beam 30 . In contrast, in multi-fiber light guides, a single window with a sufficiently large diameter is sufficient, allowing higher sensor and/or light source densities within the same area. This provides higher integration for optical sensing.

In general, one preferred implementation of sensor structure 20 is an optical heart rate monitor. Additionally, the sensor structure 20 may be a wearable device, such as a smartwatch. For wearable devices, for example in the form of wristwatches with optical heart rate monitors, the housings are typically secured to the body with a window or faceplate, respectively, and come into contact with the skin. Wearable devices are particularly susceptible to shock, for example when the device is removed and placed on a hard surface. However, thanks to the chemically strengthened abutment surfaces, the likelihood of fracture, or at least chipping of the outer surface, due to the stress of impact is reduced. A multi-fiber light guide with a glass member 9 according to this disclosure is also significantly more durable than the plastic windows frequently used for wearable devices. These plastic windows degrade over time and have performance issues with respect to waterproofness, durability and aesthetics.

In the above example, the multi-fiber light guide 1 is shaped like a plate. However, it is also conceivable to use an elongated light guide with a length of the guiding fiber 3 longer than the longest lateral dimension of the glass member 9 . One example is a dental rod. These rods are used to direct light into the treatment area and cure photopolymerizable fillers for dental treatments, for example.

A dental rod 35 as a further embodiment of the multi-fiber light guide 1 is shown in FIG. Dental rod 35 is shaped as an elongated light guide in which the length of guiding fiber 3 is longer than the longest lateral dimension of glass member 9 . In the figure, a single guiding fiber 3 is drawn as a diagonal line to illustrate the orientation and placement of the guiding fiber. The shell 17 as well as the mating surfaces 10, 11 of the multi-fiber light guide 1 are chemically strengthened and the glass member 9 is stabilized to withstand impact shock. For example, reinforcement can help avoid breakage if the dental rod-equipped instrument is accidentally dropped.

Furthermore, the embodiment of FIG. 8 is an example of a further refinement of the multi-fiber light guide 1 . According to a first refinement, the two abutment surfaces 10, 11 can differ in their cross section. This difference may include at least one of the surface area and shape or contour of the mating surfaces 10,11. In the above examples, the cross-sectional surface areas differ in that the surface area of the abutment surface 11 is smaller than the surface area of the opposing abutment surface 10 . The multi-fiber light guide 1 can therefore be tapered, as is also realized in the example depicted.

Furthermore, in general the multi-fiber light guide 1 may contain at least one bend. Although the above features can be realized with plate-like multi-fiber light guides, they are generally more effective with elongated light guides 1 . Both the bending and the taper of the glass member 9 can be produced by hot forming. The taper is produced by heating the glass member and pulling it axially. Thus, the number of fibers 3 remains constant. However, the fiber diameter decreases along the taper.

Irrespective of the various shapes of light guide 1 discussed within this application, multi-fiber light guides can be manufactured using a drawing process starting from a suitable preform. Repeated drawing results in a continuous reduction in fiber diameter resulting in fibers having small, especially microscopic, lateral dimensions.

Specifically, the multi-fiber light guide 1 can be manufactured by a method comprising the following steps, which are also explained using FIGS. 9-12:
- providing a preform 40 having at least one rod 41 of a first glass and at least one glass member 43 of a second glass having a lower refractive index than the first glass, the preform 40 comprising: assembling such that the rod 41 is surrounded by said second glass;
- heating the preform to fuse the rods 41 to at least one glass member 43; and - stretching the preform 40 to increase its length and decrease the lateral dimensions of the rods 41;
- cutting the stretched preform 40 into portions 47; and - assembling said portions to form a further preform 40, where the portion 47 is assembled into a new preform, the new preform. and cutting the drawn preform 40 into portions 47, preferably at least once.
- cutting a portion 47 from the preform 40 to obtain a glass member 9 as described herein comprising a number of light guiding fibers 3 having a core of the first glass surrounded by a cladding of the second glass; ,
The glass member is immersed in a salt bath 50 containing alkali ions to exchange ions with the alkali ions in the glass of the glass member 9, and the ion exchange layer 13 that applies compressive stress to the glass is formed on the surface of the glass member. Forming stage.

Preferably, the coefficient of thermal expansion (CTE) of the first glass, in particular the desired core 5 glass, is different from the CTE of the second glass, in particular the desired cladding 7 glass. The CTE of the first glass, in particular the desired core 5 glass, is smaller than the CTE of the second glass, in particular the desired cladding 7 glass, in order to prevent mismatches, especially tensile stresses during further stages. Advantageously, the CTE of the first glass, in particular the desired core 5 glass, is greater than the CTE of the second glass, in particular the desired cladding 7 glass, in particular to generate at least a slight compression. The difference between the CTE of the first glass and the CTE of the second glass is specified as CTE _{(cladding/core)} =CTE _{second glass} -CTE _{first glass} difference. According to this specification, a CTE _{(cladding/core)} difference of +3.5 ppm/K to -1 ppm/K is preferred. A positive value results in compression of the cladding 7 glass, while a negative value results in an increase in the extensibility of the cladding 7 glass.

A CTE _{(cladding/core)} difference of less than 3 ppm/K is preferred to achieve sufficient compression. If the glass member 9 includes a shell 17, it is preferred that the CTE of the glass of the cladding 7 be less than the CTE of the glass of the shell 17 to prevent mismatch, especially tensile stresses. Advantageously, the CTE of the glass of the shell 17 is greater than the CTE of the glass of the cladding 7, especially in order to generate at least a slight compression. Therefore, the CTE of the shell 17 glass is different from the CTE of the cladding 7 glass. The difference between the CTE of the shell glass and the CTE of the cladding glass is specified as CTE _{(shell/cladding)} =CTE _{shell 17 glass} -CTE _{cladding 7 glass} difference. According to this specification, a CTE _(shell/core) difference of +5 ppm/K to -1 ppm/K is preferred. A positive value results in compression of the shell 17 glass on the cladding 7 while a negative value results in an increase in extensibility of the shell 17 glass.

FIG. 9 shows the preform 40 in cross-section. The glass member 43 can be a single member having a through hole therein into which the rod 41 is inserted. Alternatively, multiple glass members 43 positioned between the rods 41 and covering the outward facing sides of the rods 41 can be used.

Rod 41 is fused to one or more glass members 43 to stretch preform 40 to increase its length and decrease the lateral dimension of rod 41 . Fusing can in particular be done by a drawing step, which also requires heating to soften the glass. FIG. 10 shows the stretched preform. The ends 45 of the preform 40 still have their original lateral dimensions because the preform is gripped at its ends to apply the stretching force. The stretched portion is then cut into portions 47 . The parts are brought together to form a further preform 40 as shown in FIG. The steps of assembling and drawing the preform 40 and cutting the drawn preform 40 into portions 47 can be repeated until the desired fiber dimensions are reached. A portion 47 is then cut from the drawn preform, which is the glass member 9 as described above. Portion 47 may be a lamina, which forms a faceplate as exemplarily shown in FIGS. 1 or 7, or may be stretched as in the example of FIG. Finally, the glass member 9 is immersed in a salt bath 50 within a container 51, as shown in FIG. Suitable for strengthening glass members with sodium-containing glasses is a molten salt bath containing molten potassium nitrate.

According to a variant of said method, the multi-fiber light guide 1 can be further processed before chemical strengthening. In particular, the multi-fiber light guide 1 is fixed or placed in the socket to obtain an assembly with the multi-fiber light guide 1 fixed in the socket. After bonding, the assembly with the multi-fiber light guide 1 and the socket is subjected to alkali ion exchange to chemically strengthen it. A composite 55 having a socket 53 and a multi-fiber light guide 1 secured thereto is shown in FIG. Of course, according to an alternative embodiment, chemical strengthening of the multi-fiber light guide 1 may be carried out before manufacturing the assembly. If the multi-fiber light guide 1 is chemically strengthened while being anchored in the socket 53, ion exchange can be suppressed in the part of the multi-fiber light guide that is anchored in the socket 53 and thus covered. Conversely, when using a glass solder with an alkali oxide content, the glass solder can also be subjected to an ion-exchange process and exhibit an ion-exchange layer. Therefore, it can be verified from the assembly 55 whether the multi-fiber light guide 1 is fixed to the socket 53 before or after chemical strengthening.

Socket 53 may be metal, preferably stainless steel, ceramic or other glass. In one embodiment, socket 53 is a metal ring. However, the socket 53 may also be a housing part, for example a housing part of a housing for a wearable electronic device. According to a further embodiment, which is also realized in the depicted example, the multi-fiber light guide 1 is soldered to the socket 53 . The solder 57 between the multi-fiber light guide 1 and the socket 53 may be glass solder.

REFERENCE SIGNS LIST 1 multi-fiber light guide 3 fiber 5 core of fiber 3 7 clad 9 glass member 10, 11 mating surface of light guide 1 12 side of light guide 1 13 ion exchange layer 15 absorbing fiber 17 shell 20 optical sensor structure 22 optical sensor Housing of structure 20 24 sensor element 26 light source 28 printed circuit board 30, 31 light beam 32 interaction area 35 dental rod 40 preform 41 rod 43 glass member 45 end of 40 47 cut from drawn preform 40 part 50 salt bath 51 container 53 socket 55 assembly 57 solder

Claims (15)

A multi-fiber light guide (1),
- comprising a number of light guiding fibers (3), each said fiber (3) comprising:
- comprising an elongated glass core (5), wherein:
- said core (5) is surrounded by a glass cladding (7),
- said cladding (7) forms a rigid continuous glass member (9) with said core (5), and
the core (5) has a higher refractive index than the cladding (7) so that light can be guided by total internal reflection along the glass core (5) and the multi-fiber light guide (1 ) has two abutting surfaces (10, 11) and the core (5) terminates on both abutting surfaces (10, 11) so that light can enter the core (5) can be guided from one abutment surface (10) to the other abutment surface (11) along
- the glass of the core (5) and the glass of the cladding (7) contain alkali ions, and
the alkali ions of said glass are at least partially exchanged by higher atomic number alkali ions in an ion exchange layer (13) at each said abutment surface (10, 11), said ion exchange layer ( A multi-fiber light guide (1), wherein exchanged alkali ions within 13) impart compressive stress at said abutment surfaces (10, 11). The multi-fiber light guide (1) has at least one of the following characteristics:
- said ion exchange layer (13) also extends along a shell (12) of said glass member (9) extending between said two abutting surfaces (10, 11);
- the penetration depth of the ion-exchange layer (13) in the glass core is different from the penetration depth of the ion-exchange layer (13) in the glass cladding,
- the coefficient of thermal expansion (CTE) of the core (5) is different than the CTE of the cladding (7);
- the CTE of the shell (17) glass is different from the CTE of the cladding (7) glass,
A multi-fiber light guide (1) according to claim 1, comprising: A multi-fiber light guide (1) according to claim 1 or 2, characterized in that said glass member is a disc having a disc thickness smaller than its shortest lateral dimension. 3. A multi-fiber light guide (1) according to claim 1 or 2, wherein said multi-fiber light guide (1) has an elongated shape and the length of said light guiding fibers (3) is greater than the longest lateral dimension of said glass member (9). Multi-fiber light guide (1). At least one of the following characteristics:
- said abutting surfaces (10, 11) differ in their surface area,
- said abutment surfaces (10, 11) differ in their shape,
- said glass member (9) comprises at least one bend;
A multi-fiber light guide (1) according to claim 4, comprising: At least one of the following characteristics:
- said cores (5) are arranged in a hexagonal pattern,
- the glass of said cladding (7) is light absorbing,
- said glass member (9) of said multi-fiber light guide (1) comprises a light absorbing fiber (15);
A multi-fiber light according to any one of claims 1 to 5, characterized in that the center-to-center distance of adjacent light guiding fibers (3) is between 2 μm and 500 μm, preferably at most 200 μm. Guide (1). A multi-fiber light guide according to any one of claims 1 to 6, wherein said glass member (9) comprises a glass shell (17) surrounding a cladding (7) with an embedded core (5). 1). An assembly (53) comprising a multi-fiber light guide (1) according to any one of claims 1 to 7, secured in a socket (53). An assembly according to any one of claims 1 to 8, wherein said multi-fiber light guide (1) is chemically strengthened while being fixed in a socket (53). An optical sensor structure (20) comprising a sensor element (24), said sensor element (24) being a housing comprising a multi-fiber light guide (1) according to any one of claims 1 to 7. (22) said optical sensor structure (20) encased within to direct light from outside said housing (22) to said sensor element (24); A light source (26), preferably a semiconductor light source, is further included in said housing (22), wherein said light source (26) and said sensor element (24) are connected to said multi-fiber light guide (1). 11. The optical sensor structure (20) of claim 10, wherein (26) emits light through said multi-fiber light guide and said sensor element is arranged to receive light through the same multi-fiber light guide. An optical sensor structure (20) according to claim 10 or 11, wherein said optical sensor structure (20) is an optical heart rate monitor. Optical sensor structure (20) according to any one of claims 10 to 12, wherein the optical sensor structure (20) is a wearable device. A method for manufacturing a multi-fiber light guide (1) according to any one of claims 1 to 7,
- providing a preform (40) comprising at least one rod (41) of a first glass and at least one glass member (43) of a second glass having a lower refractive index than said first glass; and assembling said preform (40) such that said rod (41) is surrounded by said second glass;
- heating said preform to fuse said rod (41) to said at least one glass member (43); and - stretching said preform (40) to increase its length and said reducing the lateral dimension of the rod (41);
- cutting the stretched preform (40) into portions (47); repeating the steps of assembling into a form, stretching a new preform (40), and cutting the stretched preform (40) into portions (47), preferably at least once,
- cutting a portion (47) from said preform (40) to yield a number of light guiding fibers (3) having a first glass core (5) surrounded by a second glass cladding (7); obtaining a glass member (9) comprising
An ion-exchange layer in which the glass member is immersed in a salt bath (50) containing alkali ions to exchange ions with the alkali ions in the glass of the glass member (9) to apply compressive stress to the glass. forming (13) on the surface of said glass member (9). 15. The multi-fiber light guide (1) is fixed in a socket (53), and the assembly (55) comprising the multi-fiber light guide (1) and the socket is chemically strengthened after fixing. The method described in .

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